Cysts on the skin, mainly sebaceous cysts or or cystic acne. Sebaceous cysts are caused by sebum accumulation due to blockage of the sebaceous gland ducts. The clinical presentation is usually a localized dilatation of the follicular opening, and when there is sebum blocking the follicular opening and oxidized by the air, it may present a blackhead-like appearance. The contents of the cyst are a mixture of sebum secreted by the sebaceous glands, the shedding of cells from the follicular wall, and microorganisms from within the follicle. Cystic acne is associated with localized skin flushing, swelling, and more severe pain, and the contents are a purulent discharge of sebum, bacteria, and necrotic tissue.